If you remove a vacuum line and spray in a little gasoline, carb cleaner, ether or whatever your favorite combustible is, will it fire right up when cold. I note that you've "bench" tested the cold start valve but that doesn't mean it's working when cranking the motor. I've seen more than a few times the cold start timer/temp sensor loosing it's ground to the motor despite being tight mechanically. You'll find that sensor/timer on the exhaust side of the motor towards the rear of the block. It screws into an intermediate fitting which then screws into the block. Remove the works, clean all the threads and electrical connections and reinstall. Check the corresponding terminal at the starter too. I'm betting that's where the problem lies--of course it doesn't cost me anything to say so.